Yakage is a manga only character, that is a character that has only appeared in Hitoshi Okuda's No Need For Tenchi! and The All-New Tenchi Muyo! manga series.
There is not much known about him since he is a one time only character in the Manga. But there is a hint that he is from Jurai since like Ayeka he had two guardians made of wood but had different design by the names Kuze & Hakkou that communicated by beeps instead of words. Another hint was when he paid respect to Princess Ayeka. There was more a hint more like a conformation that he was from Jurai when Washu made the comment about how she know that people for Jurai and that she knows that they have young live but asked herself if he was that young.
The two guardians were the ones that lead the attack on the house by his orders after he recovered Minagi. He was not a real bad guy, but he was not good either since he attacked the house. The conflict with him forced Tenchi to learn how to become more one with the Tenchi-Ken after Ayeka was kidnapped.
He attacks the house sometime after the incident with Kagato but just for a chance to face Tenchi and his sword. Washu stated that he was at one a helper in her lab which explains how come he has access to Ryoko's DNA. He created Minagi using that DNA and his own according the manga. In the manga, he stated that he was there when Ryoko was created when he looked at her after she asks who he was and stated "You do not remember, do you?".
His goal in life was to create the "Ultimate Sword" to the point that he wanted to face Tenchi. He stated this was due to the fact that the sword Tenchi-Ken was the strongest known. He found out about its located due to Kagato's defeat. he was in the rush due to the fact that he was dying since he was the last of a long line of clones.He had trained Minagi in the art of sword usage and planned to leave his created sword with her. Minagi who was born as part of Yakage's plan to create the "Ultimate Sword", that could surpass the Tenchi-ken; and make Minagi the owner of his sword was the one that defeated Yakage. She did this because she lost her memory and found that he was connected to him and decided to fight him in order to recover it. He is considered as her master as well father.